Currently, approximately half of the targets of drug development are G protein-coupled receptors (GPCR) characterized by a seven transmembrane domain structure. G protein-coupled receptor TGR23 is a receptor overexpressed in cancer cell lines such as human colon cancer cells LS 174T, LS 180 and SW 403, human gastric cancer cell KATOIII, etc. and colorectal cancer tissues. As ligand peptides for the receptor, there have been found a polypeptide having the amino acid sequence represented by SEQ ID NO: 1 (hereinafter sometimes briefly referred to as human TGR23-2 ligand), a polypeptide having the amino acid sequence represented by SEQ ID NO: 2 (hereinafter sometimes briefly referred to as rat TGR23-2 ligand), a polypeptide having the amino acid sequence represented by SEQ ID NO: 3 (hereinafter sometimes briefly referred to as mouse TGR23-2 ligand), and the like (WO 02/31145).
In order to further clarify the physiological functions of human TGR23-2 ligand, neutralizing antibodies to human TGR23-2 ligand, rat TGR23-2 ligand or mouse TGR23-2 ligand and a simple assay system for detecting/quantifying these TGR23-2 ligands with high sensitivity have been earnestly desired.